TRAINING MANUAL
CALIBER .50, М2
BROWNING MACHINE GUN
THIS MANUAL IS
IDENTIFIED AS
FGA
.AC SPARK PLUG DIVISION, General Motors Corporation
". FblMT, MICHIGAN, V. S. A.
FRXGIDAXRE DIVISION, General Motors Corporation
Index
Introduction ...	1
General Description	2
Specifications	...	............ 8
General Functioning	...	...	........ 10
Functions and Details	... 12
Detailed Functioning	...	........ 26
Safety Rules	....	.39
Disassembly and Assembly	............... 40
Changing Feed	....	... 53
Headspace Adjustment ................................................................ .55
Manipulation	....	.	. 60
Malfunctions	 62
Checking Before and	After Flight ..	  67
Maintenance	 68
Ammunition	. .	69
Complete Parts List	... 72
Heavy Barrel, М2, Gun	. . 80
Water Cooled, М2, Gun	82
Disassembled Parts of Complete Gun (Extended Page*)	....	83
*It will be helpful, when studying this manual, to spread out this page for reference
FOREWORD
This book has been prepared for the Ordnance Department in connection
with the training course being conducted by General Motors Corporation.
Introduction
The Browning Machine Gun, caliber .50, М2, aircraft basic, is a highly efficient automatic
weapon built to precision standards. It is an air-cooled, alternate feed gun and, as such, may
be fed from either the right or left side. It may be mounted on either a rigid or flexible mount.
Manual cocking and loading of the gun is necessary before it is ready to function automatically.
Then it may be fired by a mechanical or electrical accessory or by a manual trigger and
trigger bar. While firing, all mechanical action is automatically performed by the gun itself
and it will fire between 750 and 850 rounds per minute.
To care for the gun properly, so it will fire and keep on firing when needed, it is essential that
its mechanical action be thoroughly understood and this manual is published for that purpose.
The general appearance of this gun may be noted in Figure 1, which shows a top and side
view of the complete aircraft basic gun.
Top View
Side View
Figure 1 Browning Machine Gun Caliber .50, М2, Aircraft Basic
General Description
RECEIVER AND BARREL JACKET
GROUP
The receiver and barrel jacket form the main exterior portions of the gun, and in gun opera-
tion are stationary or non-recoiling. These separate assemblies may be noted in Figure 2 and
their relative position in the gun may be seen in Figure 1.
Receiver Assembly
Figure 2 Receiver and Barrel Jacket Group
2
General Description
BACK PLATE GROUP
The back plate is assembled to the rear of the receiver, forming an end cover. See Figure 3
and Figure 1.
Figure 3 Back Plate Group
3
General Description
BOLT GROUP (alternate feed)
The driving spring runs lengthwise of the gun with one end resting against the back plate.
The bolt group is housed in the upper forward portion of the receiver, and slides backward
and forward during operation. See Figure 4.
Top View
Back End View
Right Side View



Driving Spring Assembly
Figure 4 Bolt Group (Alternate Feed) and Driving Spring
4
General Description
OIL BUFFER BODY AND OIL
BUFFER GROUP
The oil buffer body and oil buffer group are contained in the lower rear portion of the receiver.
The bolt slides over the top of the oil buffer body during the back portion of the stroke. See
Figure 5.
OIL BUFFER
Top View
Right Side View
Figure 5 Oil Buffer Body and Oil Buffer Group
5
General Description
BARREL AND BARREL EXTENSION
GROUP
The barrel extension is screwed onto the breech end of the barrel to form a single unit, and the
barrel slides inside of the barrel jacket. Thus the normal position of the barrel extension is in
the lower forward portion of the receiver. See Figure 7. The bolt slides in grooves of the barrel
extension.
Figure 6 Barrel
Top View
Right Side View
Figure 7 Barrel Extension Group
6
General Description
COVER AND BELT FEED
GROUP
Figure 8 Cover and Belt
Feed Group
The cover and belt feed group is
hinged at its forward end to the top
front portion of the receiver. See
Figure 8 and Figure 1.
Specifications
BROWNING MACHINE GUN, CALIBER .50, М2
Aircraft Basic
GENERAL DATA (approximate)
Weight of Gun
Weight of Bullet
Weight of Powder Charge
Weight of Cartridge
Weight of 100 Cartridges in Metallic Links
Weight of Barrel
Length of Barrel
Number of Lands
Twist—Right Hand
Overall Length of Gun
Muzzle Velocity
Rate of Fire
Maximum Range
61 lbs.
750 grams (1.71 oz.)
200 grains (0.46 oz.)
4.20 oz.
30.25 lbs.
10 lbs.
36 inches
8
1 turn in 15 inches
56.125 inches
2,750 ft. per sec.
(1,875 mi. per hr.)
750 to 850 rounds per minute
7,200 yards (4.1 miles)
«««иг:
Figure 8A Ammunition Belt Links
Single cartridges or rounds are first assembled into a series of nested links as shown in Figure
8A. This forms a flexible ammunition belt with a single unused link trailing on one end and a
double used link on the other. The double link end is fed into the gun.
8
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General Functioning
The Browning Machine Gun, Caliber .50, М2, is an air-cooled gun capable of attaining a
rate of fire between 750 and 850 rounds a minute.
Although this gun is an automatic weapon, it is necessary to "cock" it manually to start the
operating sequence. Assume that the gun is cocked, and that the first cartridge is in its firing
position in a chamber in the rear portion of the gun barrel.
When any cartridge is fired, the burning powder violently generates gas which, since it is
confined by the cartridge case and barrel, exerts a tremendous pressure. This pressure reaches
50,000 pounds per square inch, and since this pressure pushes against the back face of the
bullet, which up to this moment is still within the cartridge case and which has a diameter of
one-half inch, a driving force of five tons pushes the bullet out of the barrel. This same force
tries to drive the cartridge case out of the chamber toward the rear. Such action is
prevented by having the bolt positively locked against the rear of the cartridge at the instant
of firing.
Figure 9A Cutaway View of Gun
10
General Functioning
When the cartridge is fired the force of recoil carries the barrel, barrel extension, and bolt
(known as the recoiling portion) backward a short distance. See Figure 10A. This motion
unlocks the bolt from the barrel and barrel extension, and the bolt throws back farther toward
the rear against a spring. This spring serves to drive the bolt forward again. The empty case is
withdrawn by the bolt from the barrel chamber and the next cartridge is extracted from the
supply belt.
The long rearward motion of the bolt is checked and as it surges forward the empty case is
ejected and the next cartridge is moved into the barrel chamber. See Figure 10B. The short,
rearward motion of the barrel and barrel extension is checked by the oil buffer and its spring;
this buffer spring serves to drive them forward again. This motion locks the bolt to the barrel
extension, thus again preventing the case from being driven toward the rear. The last forward
motion of the bolt and barrel causes the firing pin to strike the cartridge, providing a means
of releasing the sear is furnished. This cycle continues as long as trigger action is maintained
and as long as ammunition is supplied.
Recoiling Portion
Figure 10A
Figure 10B
11
Functions and Details
BACK PLATE GROUP
The back plate, besides enclosing the back end of the receiver, also houses the final bolt recoil
cushioning parts. See Figure 11. It also acts as a stop for the oil buffer group. The plate slides
downward into grooves in the receiver side plates and is latched in place with a latch and
latch lock.
Assembled and compressed into the projecting cylindrical portion is a stack of lightly greased
fiber discs which are headed on the forward end by the buffer plate. The final movement of
the bolt is stopped by the fiber discs as the bolt strikes this buffer plate, and these discs assist
in starting the forward movement.
Although the back end of the driving spring group is retained during assembly by inserting
the driving spring rod retaining pin into a hole in the receiver, in actual operation the force
of the spring is counteracted by the back plate.
The back plate furnished with the basic gun is intended for fixed mount, remote firing applica-
tions. If the gun is to be used in a flexible manner, the necessary handle and manual trigger
parts are added. An auxiliary filler piece is supplied with the basic gun to close the opening
provided for a manual trigger.
Figure 11 Back Plate Details
12
Functions and Details
BOLT GROUP (alternate feed)
Figure 12 Bolt Group (alternate feed)
Figure 13 Firing Mechanism
The bolt group is probably second in importance to the receiver. It holds the cartridge firmly
in the chamber when it is fired; it withdraws the empty case and ejects; it extracts a fresh
cartridge from the belt and inserts it in the chamber; it actuates the belt feed mechanism.
The sear mechanism, when actuated by a trigger, trips the cocked firing pin, causing the
gun to fire.
As may be noted in Figure 12, the sear, which moves vertically in the back end of the bolt,
may be depressed by pushing down on the small protrusion which extends up beyond the top
of the bolt. When a manual trigger is supplied, the sear is depressed in this manner by means
of the trigger bar in the top of the receiver. The downward motion of the sear unhooks or
releases the firing pin extension, Figure 13. This extension, along with the firing pin, snaps
forward under the action of the cocked or compressed firing pin spring, and the tip of the
firing pin protrudes from the front end of the bolt, thus striking the cartridge primer.
13
Functions and Details
BOLT GROUP (alternate £eed)
Figure 14 Back End
of Bolt
Figure IS Cocking Mechanism
The sear may also be actuated by side pressure on the end of the sear slide, Figure 14. This
slide may be assembled either right or left hand, and suitable openings in both side plates of
the receiver permit the gun to be fired from either the right or the left side. The necessary pres-
sure may be supplied by electrical (solenoid) means or by a cable attachment which fastens
to the side plate.
Although the sear is constantly being forced upward by the sear spring, it is retained in its slot
by the sear slide. See Figure 14. The sear stop pin projects downward through the body of the
bolt to act as a stop for the firing pin spring. See Figure 15.
The cocking lever, which at its lower end engages with a slot in the firing pin extension, has
its top end projecting above the bolt. This top end engages with a cam in the top plate bracket
of the receiver to cock the firing pin.
14
Functions and Details
BOLT GROUP (alternate feed)
^DRIVING SPRING ROD AND SPRING ASSEMBLY—C643O5
Figure 16 Bolt Group Details
The top surface of the bolt has two diagonal ways or grooves which act as cams to actuate the
belt feed mechanism in the cover. See Figure 16. The bolt switch fits into the circular depression
on top of the bolt and may be assembled to make one or the other of these two ways continuous,
the selection depending on whether ammunition will be fed from the right or left side.
The extractor which fits into a circular hole on the left-hand side of the bolt, withdraws a
cartridge from the belt and places it in the T-slot at the front end of the bolt. The extractor stop
pin stops the extractor in its downward swing on the counter-recoil stroke.
The ejector is fastened to the end of the extractor, and helps to position a new cartridge in the
feed-way when ammunition is being fed from the right-hand side. It also guides a new cartridge
into the chamber, and pushes the last empty case out of the T-slot.
A driving spring assembly fits into a lengthwise hole in the bolt, and is compressed by the rear-
ward motion of the bolt. After the bolt recoil has been stopped by the back plate, the spring
drives the bolt forward. This assembly actually has two springs, one nested inside the other,
and both are slipped over a rod with suitable end retaining parts.
15
Functions and Details
BARREL AND BARREL
EXTENSION GROUP
— ,!— ...
BARREL—D35348A
Figure 17 Barrel
.3 .3
Figure 18 Barrel Extension Details
The function of the barrel is to direct the discharged projectile. The rifling or grooving causes
the projectile to rotate and maintain direction and prevent tumbling.
The barrel is of one piece, threaded at the rear or breech end to screw into the barrel extension.
See Figure 17. Although the barrel tapers toward the front or muzzle end, the last portion
ground straight so as to permit it to slide in the front bearing of the barrel jacket. A chamber
formed in the barrel at the breech end which has the exact contour of the cartridge. A series
of notches or serrations is formed in the rear cylindrical outer surface. When the barrel is
screwed into the barrel extension, one end of the barrel locking spring (Figure 18) fits into these
serrations to prevent any change in the degree of engagement between the barrel and barrel
extension during firing. Should adjustment be necessary (described later) it can readily be
made against the tension of the barrel locking spring.
16
Functions and Details
OIL BUFFER BODY AND OIL
BUFFER GROUP
Figure 22 Accelerator
Figure 21 Breech Lock Depressor
On the recoil or rearward stroke of the barrel extension the breech lock pin is engaged by the
breech lock depressors which are riveted to the oil buffer body. See Figure 21. The depressors
cause the breech lock to unlock the bolt from the barrel extension.
The accelerator is assembled into the forward portion of the oil buffer body. See Figure 22.
On the recoil stroke it assists in driving the bolt to the rear. See Figure 22. During the rearward
or recoil stroke the claws on the accelerator bear against the shoulders on the barrel exten-
sion shank, thus locking and preventing the barrel extension from moving forward on the
counter-recoil stroke until the bolt strikes and moves the accelerator forward. Thus the locking
movement of the breech lock is timed so as to bring the lock up exactly when the notch in the
bolt is in position.
A tube lock in a groove in the bottom of the oil buffer body (see Figure 23) exerts pressure
against the bottom of the accelerator to keep it in the locked position until released by the bolt.
The back end of the lock projects beyond the buffer body in such a way that a bevel on the
bottom edge of the back plate holds the lock forward. This insures the lock will hold the acceler-
ator very firmly in the locked position; thus the main body of the bolt will pass over the acceler-
ator without interference until the back lug on the bolt actually strikes the accelerator.
18
Functions and Details
OIL BUFFER BODY AND OIL
BUFFER GROUP
Figure 24
Oil Buffer Cutaway
Figure 25
Oil Buffer Details
The oil buffer serves to absorb and partially store the recoil energy of the barrel and barrel
extension during the recoil stroke. See Figure 24. This stored energy is given up by the oil buffer
spring to drive the barrel extension and barrel forward. The shock absorbing action of the
spring is supplemented by a piston and oil cylinder in the rear of the oil buffer spring. See
Figure 24 and Figure 25. The degree of oil leakage across the piston on the recoil stroke controls
the rate of fire. This may be adjusted manually.
The piston rod and head may slide but are prevented from rotating by the guide key seating
in the slot in the oil buffer body. See Figure 25. The piston valve may be rotated to change
the leakage aperture at the edge of the piston by turning the oil buffer tube against the restrain-
ing action of the lock spring in the tube serrations.
A relief valve in the oil buffer tube cap permits some oil to escape on the initial recoil stroke as
the piston rod crowds into the oil filled cylinder. It also allows for oil expansion due to tempera-
ture rise.
19
Functions and Details
RECEIVER AND BARREL
JACKET GROUP
Figure 26 Receiver Aeeembly
Figure 27 Right Side View of Gun and Receiver
The receiver is probably the most important portion of the gun since it is the "backbone" or
main strength member. As such it includes the mountings by which the gun is supported. See
Figure 26 and Figure 27. In addition it forms a strong, accurate housing to protect and position
the working parts of the gun. It also contains a part of, and supports the remainder of, the
ammunition feeding mechanism. It is further utilized to support the various types of trigger
mechanisms which are necessary for the different services to which the basic gun may be
adapted (i.e., fixed mounting, remote firing as in airplane wing installations, or flexible mount-
ing, manual trigger as in tanks). The barrel jacket is supported by the receiver.
The receiver is made of two steel side plates riveted at their forward portion to a trunnion block;
with top and bottom plates riveted to the side plates toward the rear. On top at the extreme
forward portion of the receiver a trunnion block cover protects the sight grooves until such time
as a sight is installed.
20
Functions and Details
RECEIVER AND BARREL
JACKET GROUP
Figure 28 Receiver Details
Directly below the trunnion block cover is a detent pawl which meshes with the cover to retain
same in one of three open or raised positions. See Figure 28. The top front of the receiver is open
to permit access to the bolt and belt feed mechanism.
Riveted to the underside of the top plate is the top plate bracket which supports the trigger
bar pin on which the trigger bar pivots. The trigger bar is assembled in all caliber .50 basic
guns even though some applications which demand firing from remote position do not use this
piece. Front and rear trigger bar stops are also provided. The top plate bracket has suitable
cams for engaging the cocking lever of the bolt. Riveted to the underside of the top plate is the
bolt latch bracket which, although part of all basic guns, is used only on those applications
which require single shot guns.
The rear of the receiver is slotted to receive the back plate.
The bottom plate carries the breech lock cam which because of a machined shoulder "floats"
slightly when bolted down. The breech lock cam forces the breech lock up into the bolt recess,
acts as a forward stop for the oil buffer body, and has grooves or ways in which the barrel
extension rides.
The bottom front portion of the receiver is open to permit empty cartridge cases to be ejected.
A switch is pivoted on the inside of the left side plate, with a hairpin spring recessed in the
plate under the switch.
21
Functions and Details
RECEIVER AND BARREL
JACKET GROUP
LEFT HAND SIDE PLATE ASSEMBLY—C4O85
Figure 29 Receiver Details (Front Portion)
The front end of the receiver is formed by the trunnion block which is threaded to fit into the
trunnion adapter. See Figure 29. A suitable shim is inserted between the trunnion block and
adapter so that the adapter when screwed on tightly will position to line up with the other
mountings.
The side plates are notched at the top front portion so that a cartridge belt may be fed into the
gun from either side. At these notches the belt holding pawl brackets are riveted to each side
plate. These brackets support the belt holding pawl and the cartridge stops, and are so built
that parts may be assembled on either right or left side to permit feeding ammunition from
either the right or left side. The link stripper and rear cartridge stop are used for right-hand
feed only. The rear right-hand cartridge stop assembly is used for left-hand feed only. The
cartridge aligning pawl, which is part of this cartridge stop assembly helps to position a
cartridge in the feedway when ammunition is being fed from the left-hand side.
22
Functions and Details
RECEIVER AND BARREL
JACKET GROUP
Figure 30 Barrel Jacket Details
The barrel jacket is perforated to permit air to blow through onto the barrel for cooling purposes.
See Figure 30. The jacket is stationary, and prevents any object from interfering or rubbing
against the barrel which must move during firing. It screws into the trunnion and is locked in
place with a small set screw. The front barrel bearing is screwed into the front end of the barrel
jacket and is locked in place with two small screws.
23
Functions and Details
COVER AND BELT
FEED GROUP
Figure 31
The cover permits access to the bolt and to the feeding mechanism, some parts of which are
located in the feedway and some on the underside of the cover. See Figure 31.
The front or hinge end of the cover is serrated so that it may be retained in one of several open
positions. A latch is built into the back end of the cover so as to lock it securely to the receiver.
This latch may be assembled right or left-hand, depending on the type of slide used.
Operating in a crosswise groove on the underside of the cover is the belt feed slide. This is
actuated by the belt feed lever, one end of which rides in the ways on top of the bolt. The belt
feed slide carries the belt feed pawl which on each stroke snaps over a new cartridge and pulls
it into position so that it may be extracted from the belt. The pawl, slide, and lever may be
repositioned to change the direction of feed. In the event that the extractor fails to extract a
cartridge from the belt, the belt feed pawl arm prevents damage to parts by holding the belt
feed pawl up so that it cannot engage the next cartridge in the belt.
The cover extractor cam is riveted to the underside of the cover. This cam forces the extractor
and new cartridge downward as the bolt travels toward the rear. The cover extractor spring
which is also assembled to the underside of the cover limits the upward movement of the ex-
tractor during the final forward motion of the bolt. See Figure 31A for complete details of cover
and belt feed group.
24
Functions and Details
COVER AND BELT
FEED GROUP
Figure 31A Cover and Belt Feed Details
25
Detailed Functioning
In the description of the detailed functioning of the caliber .50 Browning
Machine Gun which appears on the following pages, it is assumed that, first,
the ammunition belt has been properly started into the gun and the cover has
been closed and latched, second, the gun has been manually cocked and a
cartridge is in its proper position in the chamber and ready to be fired and,
third, a manual trigger and trigger bar are to be used to fire the gun.
Each time a cartridge is fired, the mechanical action within the gun involves
many parts moving simultaneously or in their proper order. To gain a working
knowledge of the operation of these parts and their relationship to each other,
the action has been separated into various phases. These are described in
the following order:
1.	FIRING
2.	RECOILING
3.	COUNTER-RECOILING
4.	COCKING
5.	AUTOMATIC FIRING
6.	FEEDING
7.	EXTRACTING AND EJECTING
26
Detailed Functioning
FIRING
Figure 32
When the gun has been loaded and the faring pin spring has been cocked or compressed
manually, the faring mechanism is as shown in Figure 32. The gun is now ready to fire.
Figure 33
When the trigger is pressed it raises the back end of the trigger bar. The trigger bar pivots on
the trigger bar pin, causing the front end to press down on the top of the sear. The sear is forced
down until the notch in the sear is disengaged from the shoulder of the firing pin extension. The
firing pin and faring pin extension are driven forward by the firing pin spring to fire the
cartridge. See Figure 33.
27
Detailed Functioning
RECOILING
Figure 34
The complete cycle of the recoiling portion of the gun, which takes place as each cartridge is
fired, consists of the recoil stroke when certain parts of the gun move rearward and the counter-
recoil stroke when these same parts move forward. At the instant of firing, the barrel, barrel
extension, and bolt, known as the recoiling portion, are in the forward position in the gun, as
shown in Figure 34.
Figure 35
Figure 36
At this time the bolt is held securely against the base of the cartridge by the breech lock which
extends up from the barrel extension into a notch in the underside of the bolt. See Figure 35.
After the cartridge explodes and as the bullet travels out of the barrel, the force of recoil drives
the recoiling portion rearward. During the first three-quarters inch of travel the breech lock is
pushed back off the breech lock cam step. See Figure 36. This permits the breech lock to be
forced down out of the notch in the bolt by the breech lock depressors engaging the breech lock
pin. This unlocks the bolt.
28
Detailed Functioning
RECOILING
Figure 37
As the recoiling portion moves toward the rear
the barrel extension rolls the accelerator rear-
ward. The tip of the accelerator strikes the
lower projection on the bolt and hastens or
accelerates the bolt to the rear. See Figure 37.
(Note breech lock completely disengaged from
bolt notch.)
The barrel and barrel extension have a total rearward travel of one and one-eighth inches at
which time they are completely stopped by the oil buffer body assembly. See Figure 38.
Figure 39
During this recoil of one and one-eighth inches the oil buffer spring is compressed in the oil
buffer body by the barrel extension shank. The spring is locked in the compressed position by
the claws of the accelerator which are moved against the shoulders of the barrel extension
shank. See Figure 39.
29
Detailed Functioning
RECOILING
PISTON VALVE PISTON ROD HEAD
Figure 40
The oil buffer assists the oil buffer spring in bringing the barrel and barrel extension to rest
during the recoil stroke, as shown in Figure 40. During the one and one-eighth inch of rearward
travel the piston rod head is forced from the forward end of the oil buffer tube to the rear. The
oil at the rear of the oil buffer tube under pressure of the piston escapes to the front side of the
piston. Its only path is through valve-restricted notches between the edge of the piston rod head
and the oil buffer tube.
Figure 41
The bolt travels rearward for a total of seven and one-eighth inches. During this travel the two
nested driving springs are compressed. The rearward stroke of the bolt is finally stopped as the
bolt strikes the buffer plate, as shown in Figure 41. Thus, part of the recoil energy of the bolt is
stored in the driving springs and the remainder is absorbed by the buffer discs in the backplate.
30
Detailed Functioning
COUNTER-RECOILING
Figure 42
After completion of the recoil stroke the bolt is forced forward by the energy stored in the driving
spring and the compressed buffer discs. When the bolt has moved forward about five inches
the tip of the accelerator is struck by a projection on the bottom of the bolt. See Figure 42.
This rolls the accelerator forward.
Figure 43
As the accelerator rolls forward the accelerator claws are moved away from the shoulders of
the barrel extension shank. This releases the oil buffer spring. The energy stored in the spring
shoves the barrel extension and barrel forward. See Figure 43
31
Detailed Functioning
COUNTER-RECOILING
c
Figure 44
No restriction to motion is desired on the forward or counter-recoil stroke of the barrel and barrel
extension; therefore, on the forward stroke additional openings for oil flow are provided in the
piston rod head of the oil buffer assembly. The piston valve is forced away from the piston rod
head as the parts move forward, uncovering these additional openings. This provides an addi-
tional path and permits oil to escape freely at the opening in the center of the piston valve as
well as at the edges of the piston valve and head next to the tube wall, as shown in Figure 44.
As the barrel extension moves
forward the breech lock en-
gages the breech lock cam
and is forced upward. The
bolt, which has been con-
tinuing its forward motion
since striking the accelera-
tor, has at this instant
reached a position where the
notch on the underside is
directly above the breech
lock, thus permitting the
breech lock to engage the
bolt. See Figure 45. The bolt
is thereby locked to the bar-
rel extension just before the
recoiling portion reaches the
tiring position.
Figure 45
32
Detailed Functioning
COCKING
TOP PLATE BRACKET
COCKING LEVER	BOLT
Figure 46
SEAR SPRING
FIRING PIN SPRING FIRING PIN EXTENSION
Figure 47
COCKING LEVER FIRING PIN EXTENSION
Figure 48
Figure 49
The act of cocking the gun is begun as the bolt starts to recoil immediately after firing. Thus the
tip of the cocking lever which is in the V-slot in the top plate bracket, as shown in Figure 46,
is forced forward.
The cocking lever is pivoted so that the lower end forces the firing pin extension rearward. The
firing pin spring is thus compressed against the sear stop pin. The shoulder at the back end
of the firing pin extension is hooked over the notch at the bottom of the sear under pressure of
the sear spring. See Figure 47.
During the forward motion of the bolt the tip of the cocking lever enters the V-slot of the top
plate bracket. See Figure 48. This action swings the bottom of the cocking lever out of the path
of the firing pin extension, as shown in Figure 49; thus permitting the firing pin to snap forward
to fire the cartridge.
When the recoiling portion is almost in the forward position the gun is ready to fire. If no trigger
action is given at this instant, the recoiling portion assumes its final forward position, as shown
in Figure 49, and the gun ceases to fire. The parts are now in the position shown in Figure 32
and the gun is again ready to fire.
33
Detailed Functioning
AUTOMATIC FIRING
SEAR	FIRING PIN EXTENSION
Figure 50
For automatic firing the trigger is pressed and held down. The sear is depressed as its tip is
carried against the cam surface of the trigger bar by the forward movement of the bolt near the
end of the counter-recoil stroke. See Figure 50. The notch in the bottom of the sear releases the
firing pin extension and the firing pin, thus automatically firing the next cartridge at the
completion of the forward stroke. The gun fires automatically as long as trigger action is
maintained and until the ammunition supply is exhausted.
34
Detailed Functioning
FEEDING
Figure 51
The belt feed mechanism is actuated by the bolt. When the bolt is in the forward position the belt
feed slide is within the confines of the gun. Figure 51 shows the mechanism as from above with
the cover removed. A stud at the rear of the belt feed lever is engaged in the diagonal groove
or way in the top of the bolt.
Figure 52
As the bolt moves rearward during recoil the belt feed lever is pivoted. The forward end of the
belt feed lever moves the belt feed slide out of the side of the gun and over the ammunition belt.
Note: Ammunition feed in Figure 52 is from the left side of the gun. Feed from either side is pos-
sible with all caliber .50, М2 guns.
35
Detailed Functioning
FEEDING
HOLDING
Figure 53
BELT FEED SLIDE
Figure 54
Figure 55
BELT
HOLDING
Figure 56
The ammunition belt is pulled into the gun by the belt feed pawl which is attached to the belt
feed slide. When the bolt is forward the belt feed pawl has positioned a cartridge directly
above the chamber. The belt holding pawl is in a raised position to prevent the ammunition
belt from falling out of the gun. See Figure 53.
As the bolt recoils the belt feed slide is moved out over the belt, and the belt feed pawl pivots so
as to ride over the next cartridge, as shown in Figure 54.
At the end of the recoil stroke the travel of the belt feed slide is sufficient to permit the belt feed
pawl to snap down behind the next cartridge in order to pull the belt into the gun. See Figure 55.
As the bolt moves forward on the counter-recoil stroke the belt is pulled into the gun by the
belt feed pawl. The belt holding pawl is forced downward as a cartridge is pulled over it, as
shown in Figure 56. When the forward stroke of the bolt is completed the belt holding pawl
snaps up behind the cartridge, as shown in Figure 53.
36
Detailed Functioning
EXTRACTING AND EJECTING
Fiaure 57
As recoil starts, a cartridge is drawn from the ammunition belt by the extractor. The empty case
is withdrawn from the chamber by the T-slot in the front face of the bolt. See Figure 57.
The empty case having been expanded by the
force of explosion fits the chamber very snug-
ly and the possibility exists of tearing the
case if the withdrawal is too rapid. To pre-
vent this and to insure slow initial with-
drawal, the top, front edge of the breech lock
and front side of the notch in the bolt are
beveled, as shown in Figure 58. Thus, as the
breech lock is disengaged, the bolt moves
away from the barrel and barrel extension
in a gradual manner.
Ficrure 59
As the bolt moves to the rear the cover extractor cam forces the extractor down, causing the
cartridge to enter the T-slot in the bolt, as shown in Figure 59.
37
Detailed Functioning
EXTRACTING AND EJECTING
Figure 60
As the extractor is forced down a lug on the side of the extractor rides against the top of the
switch causing the switch to pivot downward at the rear, as can be seen in Figure 60. Near the
end of the rearward movement of the bolt the lug on the extractor overrides the end of the
switch, and the switch snaps up to its normal position.
Figure 61
On counter-recoil the extractor and cartridge are forced farther downward by the extractor lug
riding on the under side of the switch. The cartridge pushes the empty case out of the T-slot. The
extractor stop pin in the bolt limits the downward travel of the extractor so that the cartridge,
assisted by the curvature of the ejector, enters the chamber. See Figure 61. (The ejector also
ejects the last empty case.) When the cartridge is practically chambered the extractor rides
up on the extractor cam, compresses the cover extractor spring, and snaps into the groove
in the next cartridge in the belt.
38
Safety Rules
(A)	SHOP SAFETY RULES:
1.	Make sure that the gun is securely anchored in the mountings. Never lay a gun down
where it may fall.
2.	Make sure that the chamber of the barrel and the T-slot of the bolt are free from cart-
ridges.
3.	Never cock the gun against the pressure of the driving springs with the back plate
removed from the gun.
4.	Never use a cloth or waste to apply oil to the working parts of the gun, as lint may be
left on the parts which would interfere with the operation. Oil must always be applied
with a brush.
5.	Never leave twisted ends cf locking wires or cotter pins exposed.
6.	Never alter or force any part or assembly in such a manner as would tend to make
such a part or assembly not interchangeable.
7.	Always make sure that the gun has been checked and adjusted for proper head space.
8.	Be sure that the cocking lever always points forward when the bolt is placed in the
receiver. See assembly instructions on proper method of assembling the cocking lever
in the bolt.
9.	Keep tools and bench neat and clean. Maintenance and repair of the caliber .50
machine gun requires careful workmanship, and neatness is the sign of a good
workman.
10.	After the gun has been function fired, pull the bolt back twice and raise the
cover. Check the T-slot and chamber to be sure no cartridges are still in the
gun. Release the firing pin spring.
(B)	FIRING SAFETY RULES:
1.	Before loading the gun, make sure that the bore of the barrel is clear and dry. Chamber
and bore must be free of oil.
2.	Be sure that the gun has been properly checked for head space.
3.	Before loading with live rounds, always test operation by hand using dummy cartridges.
4.	Always make sure that the cover is securely latched.
5.	Be sure that the back plate is properly in place and that the latch and latch lock are
engaged.
6.	On a "hang fire'' always wait ten seconds before raising the cover to avoid the pos-
sibility of the delayed explosion taking place after the cartridge has been removed
from the chamber.
7.	The adjusting screw of the back plate should be tightened occasionally during firing.
Excess play in the discs may result in a broken back plate.
8.	After firing, pull the bolt back twice and raise the cover. Check the T-slot
and chamber to be sure no cartridges are still in the gun. Release the firing
pin spring.
39
Disassembly and Assembly
GENERAL DISASSEMBLY:
Disassembly of the caliber .50 machine gun is carried only as far as necessary for instruction,
to clean the gun properly, or to make adjustments and repairs. Parts must not be forced into
position. To strip the gun of the main groups the cover latch is released and the cover opened.
By releasing the back plate latch lock and the back plate latch, the back plate is removed.
The driving spring rod is removed by pulling the end to the left, thus releasing the retaining
pin from the hole in the side plate. The bolt is drawn to the rear until the bolt stud is in line
with the hole in the side plate, (remove stud if a slide is being used) and the bolt is taken out
from the rear end of the gun receiver
Using the point of a cartridge through the hole in the side plate the spring lock is compressed
and the oil buffer body, barrel extension and barrel are removed by pulling to the rear. By
pressing the accelerator forward the oil buffer assembly is detached. By pressing on the head
of the piston rod the oil buffer is pushed out of the body. The oil buffer assembly should not be
stripped any further than this unless it is necessary to replace the spring because the latter is
under sufficient compression to cause serious injury. The barrel may be unscrewed from the
barrel extension.
Figure 62. Cutaway View of Gun
40
Disassembly and Assembly
TO DISASSEMBLE THE BACK PLATE GROUP:
Before the back plate is removed from its locked position in the gun, the adjusting screw of the
buffer assembly may be loosened, using combination wrench No. D-28242, by turning counter-
clockwise. Care must be taken in removing the screw to avoid losing the plunger spring and
plunger.
Release the back plate latch lock and the back plate latch, and remove the back plate from
the gun by sliding upward. When the back plate has been removed, the fiber buffer discs and
the buffer plate can be readily pushed out by pushing on the buffer plate.
With a drift, force out the filler piece pin and remove the back plate filler piece. The back
plate latch is removed by forcing out the back plate latch pin. Again care must be taken to
avoid losing the back plate latch spring. Remove the latch lock by taking out the cotter pin
and removing the pin. Detach the lower filler piece by removing the two cotter pins from the
filler piece pins and removing the pins.
TO ASSEMBLE THE BACK PLATE GROUP:
Attach the lower filler piece with the extension to the left using the two filler piece pins and
insert the cotter pins.
The latch lock is attached to the extension of the lower filler piece by inserting the pin and
cotter pin. By inserting the back plate latch spring in the latch, with the other end of the spring
in the recess in the lower filler piece, the latch is pressed forward between the thumb and fore-
finger and the pin is inserted.
The filler piece is placed in the hole and the pin is inserted. Insert the buffer plate in the tube
of the back plate with the small diameter entering first. Insert the 22 fiber buffer discs and
start the adjusting screw. Replace the back plate assembly in the gun, and, using the com-
bination wrench No. D-28242, tighten the adjusting screw up to the hole in the screw. Then
insert the adjusting screw spring and plunger and tighten the adjusting screw securelv.
41
Disassembly and Assembly
TO DISASSEMBLE BOLT GROUP (Alternate Feed):
In disassembling the bolt group the extractor is removed by rotating it upward and pulling out
frcm the bolt. The bolt switch and bolt switch stud are lifted up. The cocking lever is turned
fully backward and by pushing down on the sear the firing pin is released. The cocking lever
pin and cocking lever are removed. With the thin end of the cocking lever the sear stop is
swung out of its groove. The bolt is turned over, the sear stop pushed out of engagement and
removed from its slot. The sear is depressed and the sear slide removed. The sear and sear
spring are taken out. The firing pin extension and the firing pin then will slide out to the rear.
TO ASSEMBLE BOLT GROUP (Alternate Feed):
In assembling the bolt group the filing pin and the extension, with the notch down, are inserted
in the bolt and pushed forward until the striker projects through the small hole in the front of
the bolt. The sear spring is seated and the sear placed in its guides. In a flexible gun the square
end of the sear slide can be either to the right or the left. The slide is inserted and by pressing
down on the sear it is engaged. The sear stop is inserted, pushed down as far as it will go, and
swung into its recess in the bolt. The cocking lever with the rounded nose to the rear is placed
in position and held in place by inserting the pin. The correctness of the assembly is tested by
pressing forward on the cocking lever, to cock it, returning the lever to its rear position and
pressing down on the sear. The click of the firing pin will be heard if the assembly is correct.
The bolt switch stud is inserted, the bolt switch is placed over the stud to make the groove
marked "L" continuous if left-hand feed is desired, and the extractor inserted in the bolt, being
sure the flange is under the collar.
42
Disassembly and Assembly
TO DISASSEMBLE OIL BUFFER AND OIL BUFFER BODY:
In disassembling the oil buffer group, the oil buffer body is held bottom up in the left hand. By
pressing on the head of the piston rod the oil buffer is pushed out of the body. Raise the V-
shaped projection on the rear of the tube lock assembly out of the hole in the body by inserting
a screwdriver between the lock and the body. Then lift the tube lock latch out of the rounded
cut in the slot and force the tube lock assembly out to the rear by placing the index finger
against the tips of the accelerator and rotating the accelerator rearward. The accelerator pin
is driven out and the accelerator removed. Remove spring lock from side of buffer body unless
staked in place.
Detach the oil buffer spring by gripping the oil buffer spring guide in a vise, then push on end
of the oil buffer until oil buffer piston rod pin clears the projections on the guide. See Figure 64.
Carefully give one-quarter of a turn, which will enable pin to pass through the guide. Care
should be exercised at this point, as the spring is under considerable compression and may
cause an accident if released suddenly. Unscrew oil buffer tube cap and carefully pull out oil
buffer piston rod so as not to lose oil. To remove oil buffer piston rod head, take out oil buffer
piston head nut pin, then unscrew oil buffer piston head nut, remove oil buffer piston valve and
unscrew oil buffer piston rod head.
Unscrew the gland plug from the tube cap. Then oil buffer packing, oil buffer packing gland
washer and oil buffer packing gland spring can be taken out.
Figure 64. Oil Buffer Cutaway
43
Disassembly and Assembly
TO ASSEMBLE OIL BUFFER AND OIL BUFFER BODY:
Place packing gland plug, gland packing, gland washer, gland spring and tube cap on the pis-
ton rod in the order named. Screw gland plug into tube cap. Then screw the piston rod head onto
the piston rod, with the shoulder away from the tube cap, until the end of the shoulder is flush
with the end of the rod and the pin hole in the shoulder is in line with the hole in the rod. Place
the valve on the shoulder, with the flat face toward the piston rod head, and screw the piston
head nut, with the four castellations to the rear, onto the shoulder until the back face of the
nut is flush with the end of the shoulder. If a gage is available check the clearance between
the piston rod head and valve and adjust the nut, if necessary, to provide clearance of .05".
These parts must be properly assembled for two reasons: (1) if the clearance between the rod
head and valve is not as indicated, the gun will not function properly; (2) if these parts are
screwed too far onto the piston rod, the over-all operating length of the oil buffer assembly will
be too short. In that event, the tube will be pulled forward in the body away from the back
plate as the recoiling parts near battery position, and the oil buffer spring may fail to force the
parts forward into firing position. This is particularly true when the gun is being fired in a
vertical or highly elevated position. Insert the piston head nut pin through nut, piston head
shoulder, and rod and spread the cotter pin. Now insert the rod and its assembled parts into
the oil buffer tube, being careful to line up the valve keys with the grooves in the tube wall.
See Figure 65.
SCREW—A9361
Figure 65. Oil Buffer Details
44
Disassembly and Assembly
To assemble the spring to the oil buffer assembly, grip the oil buffer spring guide in a vise with
the key up, and place the oil buffer spring over the piston rod. Place the end of the spring
against the flat face of the guide and push on the end of the oil buffer until the pin on the end
of the rod passes through the slots in the guide. Turn the buffer one-quarter turn and allow the
pin to seat in the recesses in the guide. Before removing from the vise, check to see that the
flat surface on the piston rod is vertical and the rounded portion is to the right when looking
directly at the back end of the oil buffer.
Completely fill the oil buffer with lubricating oil of the proper specification for oil buffer use
by removing two filling screws in end of oil buffer; hold buffer upright so that these holes are
on top. Start the oil to flow from the spout or nozzle of the oiler and insert the nozzle into one of
the holes. Flow oil in until it comes out the remaining hole. Retain pressure on the nozzle while
removing it from the hole to avoid possibility of getting air into the buffer. Replace the two
filling screws.
Fully insert the oil buffer in the oil buffer body with the guide key engaging in the slot in the
buffer body. Replace the spring lock in the side of the buffer body.
In assembling the oil buffer group the accelerator is placed between the depressors with the
tips up and rounded surface to the front, then the accelerator pin is inserted, taking care that
both ends of the pin are flush with the sides of the body. The tube lock assembly is positioned
over the slot in the bottom of the body and the rear end is raised so that the V-shaped projec-
tion will clear the body. Then, with the assembly depressed into the slot, it is pushed forward
until the projection is seated in the hole, and the tube lock latch is seated in the circular cut
in the slot.
Before the complete assembly is placed in the receiver, the oil buffer must be adjusted for the
desired rate of fire as described on page 60.
45
Disassembly and Assembly
flT")-"|4Ti-|4.|4.pH|.i44^p4-|4-l^r.vut..n4.rvvu1
Figure 66. Barrel Extension Group
TO DISASSEMBLE BARREL AND BARREL
EXTENSION GROUP:
The barrel is unscrewed from the barrel extension. The locking spring may be removed by
sliding it forward out of its seat unless it is staked. See Figure 66. The breech lock pin is
pushed out and the breech lock removed.
TO ASSEMBLE BARREL AND BARREL EXTENSION GROUP:
The breech lock is assembled into the barrel extension with the bevel faces to the front and
the double bevel on the top. The pin is inserted, taking care that both ends of the pin are flush
with the sides of the barrel extension. The locking spring is replaced in the seat and the barrel
screwed into the barrel extension. It will be necessary to adjust the degree of engage-
ment between the barrel and barrel extension when the gun is reassembled. See
Headspace Adjustment page 55.
46
Disassembly and Assembly
LEFT HAND SIDE PLATE ASSEMBLY—C4O85
Figure 67. Receiver Details (Front Portion)
TO DISASSEMBLE THE RECEIVER AND BARREL
JACKET GROUP:
The belt holding mechanism may be removed from the receiver by withdrawing the belt hold-
ing pawl pin, being careful not to lose the belt holding pawl springs. See Figure 67. If the gun
is set up for left-hand feed and a rear right-hand cartridge stop is used, it is detached by
removing the belt holding pawl pin. It is dismantled by pushing out the pin with a drift. This
frees the aligning pawl and permits removal of the aligning pawl plunger and spring. If the
rear right-hand cartridge stop is not used, the rear and front cartridge stop and stripper are
removed by taking out the belt holding pawl pin.
The cover group is removed by pulling the cotter pm and withdrawing the hinge pin. The cotter
pin of the cover detent pawl is pulled to remove the detent pawl and spring.
The trunnion block cover is removed by pushing out two pins with a drift.
Disassembly and Assembly
The switch and switch spring are removed by pulling the cotter pin and taking off the nut on
the outside of the left-hand side plate.
The breech lock cam is removed by taking out cotter pin on the bottom of the receiver and
loosening nut. Take out breech lock cam bolt and cam from inside of the receiver. The top
plate cover screws are staked in place so that this usually is not removed. However, the trigger
bar is removable. To take out the trigger bar, the trigger bar pin is removed by turning handle
downward about one-half inch past the vertical position. Pull the pin out of the side plate.
The trunnion adapter is removed by releasing the trunnion block lock, pulling the cotter pin
end of the lock, and at the same time unscrewing the trunnion adapter. This also removes the
shim Withdraw the cotter pin and remove the trunnion block lock and spring.
The barrel jacket is removed by first taking out the breech bearing lock screw from the top
of the trunnion, and then unscrewing the jacket with a spanner wrench The front bearing
in the jacket ordinarily is not removed since the two screws are staked.
This leaves the receiver as the only part secured to the bench mounts.
TO ASSEMBLE THE RECEIVER AND BARREL
JACKET GROUP:
Assemble the barrel jacket to the trunnion by pulling up firmly with a spanner wrench. If this
is a new jacket, a hole for the breech bearing lock screw should then be drilled with a No. 7 drill
by mating through the hole in the top of the trunnion. Insert the breech bearing lock screw.
In reassembling the trunnion adapter to the trunnion, insert lock, spring and cotter pin. A shim
of one number greater thickness than the one previously removed should be inserted between
the trunnion and the adapter. Pull trunnion lock so that lock does not project, and screw on
trunnion adapter. If, after drawing up tightly, the cuts in the underside of the trunnion and
trunnion adapter do not align, remove adapter and shim and replace shim with next larger
number. Repeat if necessary until alignment is secured. The mounting holes in the adapter
will then align with the other mounting holes on the gun, and the trunnion block lock will
engage itself in the trunnion block adapter.
If possible, turn gun upside down to assist in installing trigger bar. The trigger bar is installed
by placing it, hump toward receiver top plate, between the top plate bracket and the bolt latch
bracket, with the sear engaging surface forward. The trigger bar pin is inserted, taking care
to match the projection on the pin with the notched hole in the left side plate.
48
Disassembly and Assembly
Push the pin through the hole in the trigger bar and into the hole in the top plate bracket. The
pin is turned backwards so as to snap the handle into the small detent recess in the side plate.
Install the breech lock cam by laying it in the receiver with the bevel up and rearward. The
breech lock cam bolt is inserted downward into the hole in the cam, the cam nut is screwed on
the bolt with the castellations up, and drawn tight. If the cotter pin hole is not in line with the
castellations when the nut is tight, try other nuts or bolts until those are found which will meet
this condition. The cam should "float" slightly when bolted down.
The switch is installed by first inserting the bent end of the hairpin spring into a small hole in
the switch recess in the side plate. The spring is snapped into the recess. The back end of the
switch is slid back into the recess, being careful to have the lug on the back of the switch ride
on top of the spring. Push the threaded protrusion through the hole in the side plate and secure
with nut and cotter pin. Try the switch to see that it pivots and snaps back into position.
The trunnion block cover is set in place and secured with two pins. The detent pawl spring is
slipped onto the cover detent pawl assembly and both are inserted into trunnion and secured
with a cotter pin. Reinstall cover by placing latch end down against receiver, turning latch,
pushing down on cover and releasing latch to catch under the top plate cover. Push hinge end
of cover down against detent pawl, and insert hinge pin from right side. Secure with cotter pin.
If the gun is to be equipped for left-hand feed, it is necessary to install a rear right-hand car-
tridge stop. This stop assembly is prepared by first inserting the spring and then the plunger in
the cartridge stop. Then the aligning pawl is hinged in the cartridge stop by inserting the
aligning pawl pin. The cartridge stop assembly and front cartridge stop are held to the right
belt holding pawl bracket by inserting a belt holding pawl pin assembly.
The belt holding pawl is installed by placing the belt holding pawl springs in the depressions
in the bracket, holding the pawl in position and inserting a holding pawl pin assembly.
49
Disassembly and Assembly
Figure 68. Cover and Belt Feed Group
TO DISASSEMBLE COVER AND BELT FEED GROUP:
Open the cover. See Figure 68 and Figure 8. Remove belt feed lever cotter pin and the belt
feed lever taking care that the spring and plunger do not fly out. The belt feed lever spring
and plunger are then taken out of the hole in the side of the lever.
Remove the belt feed slide from the cover. Push out the belt feed pawl pin and remove belt feed
pawl and belt feed pawl arm and spring from the belt feed slide. Keep spring from flying out
while doing this.
Lift end of cover latch spring out of the groove in the cover and turn this lifted end slightly so
that it rests on the extractor spring. The cover latch spring is then slid away from latch and
removed.
The extractor spring is removed by prying the spring away from the cover extractor cam near
the latch end.
The latch is taken out by removing the shaft cotter pin and washer, turning the latch shaft to
the latched position and withdrawing the shaft from the cover.
50
Disassembly and Assembly
TO ASSEMBLE COVER AND BELT FEED GROUP:
The latch is assembled to the cover by placing the latch between the pin bosses on the under
side of the cover with the keyway toward the top of the cover and with the projecting wing of
the latch against the underside of the cover. The latch shaft is inserted (from either side—
depends on type of slide to be used with gun) with the key on the shaft toward the top of the
cover. Place washer on end of shaft and install cotter pin. This should be inserted so that the
head is toward the hinged portion of the cover, and the ends must be bent sharply to avoid
interference when latching down the cover.
Install the extractor spring by hooking the slotted end under the extractor spring stud, with the
curved end away from the cover. While holding the stud end in place, press the curved end
until it rests on the cover and then slide projection into the recess in the cover extractor cam.
To install cover latch spring, lay the spring inside the cover with the enlarged hole meshing
with the latch spring stud. The bent end of the spring should be against the cover. Slide the
spring toward the latch, making certain that the latch end of the spring rides up over the
projecting wing on the latch. Snap the bent end of the spring into the groove in the cover.
Assume that the gun is to be assembled for left-hand feed. Place the belt feed pawl arm
against the side of the belt feed pawl so that the arm will be toward the rear when in the
assembled gun with the cover closed. The belt feed pawl with arm and spring is placed in the
belt feed slide and the belt feed pawl pin is pushed in place. The belt feed slide is placed in
its way or groove in the cover with the pawl end of the slide toward the side from which the
gun is to be fed.
Insert the lever plunger and spring in the upper hole in the belt feed lever (for left-hand feed)
and place lever, with the shoulder up, on the lever pivot stud in the cover. In order to do this,
have the gap in the belt feed slide in line with the cutout in the cover. Push the belt feed lever
completely down so that the toe of the lever can work to and fro in the slot provided in the
cover. Replace the belt feed lever cotter pin.
51
Disassembly and Assembly
GENERAL ASSEMBLY:
Before assembling the gun, the parts should be thoroughly cleaned, oiled, and inspected for
burrs, etc. See page 68.
To assemble the gun after it has been stripped, the barrel is screwed into the barrel extension.
The oil buffer is fully inserted in the oil buffer body with the guide key in the slot in the body.
The barrel extension is held in the left hand and the oil buffer assembly in the right. The
accelerator is held up under the shank with the index finger. The breech lock depressors are
started in the guide ways of the barrel extension and the oil buffer body is pushed forward as
far as it will go.
This complete unit then is pushed into the receiver until it is locked into position. The cocking
lever is pressed forward in the bolt and the bolt inserted in the receiver, taking care not to trip
the accelerator forward. The bolt is pushed forward until the hole is lined up with the enlarged
opening in the side plate and if a slide is to be used the bolt stud is inserted. Inserting the
driving spring rod assembly, the bolt is pushed completely forward and the pin is seated in the
side plate. The back plate is replaced, the latch lock released and the back plate locked into
position. Being sure the bolt is fully forward, the cover is closed and latched and the trigger
pressed to relieve the tension on the spring.
The action of the gun should be checked by pulling the bolt back several times to see that all
parts are functioning smoothly. The headspace must be adjusted according to instruc-
tions on page 55. Operate the gun with dummy cartridges as a final check on correctness
of assembly.
52
Changing Feed
To change the ammunition feed from left to right-hand, change over parts as follows:
1.	Feed Mechanism:
Open the cover. Remove belt feed lever cotter pin and the belt feed lever.
Change belt feed lever plunger and spring from the upper hole in the belt feed lever to the
lower. Do not replace the belt feed lever but lay on bench or table for the present.
Remove the assembled belt feed slide from the cover. Push out the belt feed pawl pin and
remove belt feed pawl and belt feed pawl arm and spring from the belt feed slide. Keep spring
from flying out while doing this. Change the belt feed pawl arm over from one side of the belt
feed pawl to the other, so that when replaced in the belt feed slide it will be to the rear in the
assembled gun with cover closed. Replace the belt feed pawl, arm, and spring in the belt feed
slide and replace the belt feed pawl pin.
The belt feed slide is always placed in its way with the pawl end of the slide toward the side
from which the gun is to be fed.
Replace the assembled belt feed slide in the cover in correct position to feed right-hand. Note
that feed pawl arm is to rear. Now replace the belt feed lever in the cover. In order to do so,
have the gap in the belt feed slide in line with the cutout in the cover. Push belt feed lever
completely down so that toe of the lever can work to and fro in the slot provided in the cover.
Replace the belt feed lever cotter pin.
53
Changing Feed
2.	The Bolt:
Remove complete bolt from the gun. Remove the extractor. Raise the bolt switch so as to be
clear of the bolt switch stud and give it a half-turn and it will be seen that the other hole in switch
will be in line with the stud. Push bolt switch down into place. The way in bolt should now be
adjacent to the mark "R" on top of the bolt.
Replace the extractor. Replace the bolt in the gun.
When necessary, reverse position of the sear slide to suit trigger motor or solenoid location.
3.	Certain Fittings in the Receiver :
Pull out the two belt holding pawl pins. Take out the front cartridge stop and change to left-
hand side. A rear cartridge stop and a link stripper (furnished as auxiliary parts for each gun)
should be placed in their grooves on left-hand side. Remove the rear right-hand cartridge stop
assembly and retain for time when left-hand feed is again desired.
Change location of belt holding pawl and belt holding pawl springs from left-hand side to
right-hand side. Replace the two belt holding pawl pins.
The changing over of component parts is now completed in order to enable the gun to be fed
with cartridges from the right-hand side. To change from right-hand to left-hand feed, reverse
the procedure.
54
Headspace Adjustment
1.	HEADSPACE SIGNIFICANCE
Figure 69
Before a machine gun is fired the headspace must be adjusted. This operation is divided into
two parts, as follows:
First, the bolt must be located along the barrel extension so that when the extension is forward
against the trunnion block the locking surfaces at the front of the breech lock and bolt recess
are firmly together, as in Figure 69.
RECOIL PLATE
Second, with the bolt held in
this correctly locked, battery
position, the barrel must be
so located that a cartridge
held in the bolt T-slot against
the recoil plate will fit snugly
in the barrel chamber. See
Figure 70.
Figure 70
55
Headspace Adjustment
If these two steps have been accomplished the bullet will be driven out of the muzzle with the
maximum force since none of the powder pressure can escape at the breech of the barrel.
When the parts of the gun have been thus adjusted the weapon is said to be correctly head-
spaced.
Note: Actually, when adjusting or checking headspace a cartridge is not inserted. For check-
ing purposes the distance between the recoil plate in the bolt and the breech end of the barrel,
as indicated at "A" in Figure 71, is measured or gaged.
Figure 72
2.	TIGHT HEADSPACE
If the bolt is "jacked" too far rearward by the barrel, tight headspace adjustment will result.
Normally as the recoiling portion moves forward in counter-recoil, the breech lock is forced
upward by the breech lock cam. However, if in making the adjustment, the bolt has been
"jacked" too far rearward by the barrel, the breech lock cannot fully enter its recess in the bolt.
As a result the breech lock cannot clear the cam, and the recoiling parts are held rearward
out of battery position, as shown in Figure 72.
If in adjustment the locking surfaces have been properly engaged but the barrel has not been
positioned to provide room for a cartridge, such tight headspace may result in:
a. Failure to fire because the bolt may not go forward far enough for the sear to be depressed,
b. Failure to feed because the extractor does not move forward far enough to grasp the cart-
ridge in the feedway.
c. Sluggish fire because of too much locking friction (particularly noticeable when pulling a
long ammunition belt).
56
Headspace Adjustment
LOCKING SURFACES	BARREL CHAMBER
3.	LOOSE HEADSPACE
If the parts have been improperly adjusted with too much headspace (barrel chamber too far
away from the face of the bolt), the bolt will go too far forward and the locking surfaces of
the breech lock and bolt recess will not be in contact at the instant of firing. Loose headspace
will cause the breech lock, locking recess, and breech lock cam to be badly battered, and will
soon render these parts unfit for service.
T-SLOT LIPS BARREL EXTENSION
If the barrel position has been improperly adjusted so that it is an excessive distance from the
bolt, not only will the locking surfaces have too much play but also the cartridge will not fit
snugly in the chamber. See Figure 74. Besides battered locking parts, such improper adjustment
may cause:
a.	Ruptured or separated cartridge case.
b.	Poor shot patterns because of escape of powder pressure at the breech.
c.	Battered T-slot Ups and broken barrel extensions due to the bolt striking the barrel extension.
57
Headspace Adjustment
4.	WHEN AND HOW TO ADJUST HEADSPACE
a.	Adjust headspace each time the gun is assembled. The adjustment should be made after
the parts have been assembled into the gun.
b.	Retract the recoiling portion about one inch.
c.	Screw the barrel into the barrel extension until the recoiling parts will not go into battery
position without being forced when the bolt is released. The recoiling parts are in battery
position when the barrel extension touches the trunnion block.
d.	Screw the barrel out of the barrel extension one notch at a time until upon being allowed
to go forward slowly the parts will just reach the battery position without being forced.
The locking surfaces will now be in firm contact.
Note: Do not retract the bolt more than one inch when determining the point at which the
recoiling parts will just go into battery without being forced.
e.	When this point is found, unscrew the barrel extension TWO more notches. Proper space
for the cartridge will now be provided.
Note: To screw the barrel into the barrel extension engage the barrel notches with a screw
driver or cartridge point and pry against the right-hand side plate. To unscrew the barrel,
pry against the left-hand side plate. If the gun is set up for left-hand feed, the rear right-
hand cartridge stop assembly should be removed to give more convenient access to the
locking notches on the barrel.
Figure 75
58
Headspace Adjustment
5.	TO CHECK HEADSPACE ADJUSTMENT
The correctness of the headspace adjustment of the cal. .50 Browning machine gun may be
quickly checked by means of gage A351211. Correct headspace may have a very small
permissible variation. The gage is therefore made with a thin end called the "GO'' portion
and a thicker end called the "NO GO" portion. When the headspace is correctly adjusted the
distance between the recoil plate in the bolt and the breech end of the barrel shall equal or
exceed the "GO” portion, but shall be smaller than the "NO GO" portion. The old style gage,
A196228, shall not be used in checking headspace of the aircraft gun.
Headspace adjustment may be checked as follows:
a.	Cock the gun by fully retracting the recoiling portion and allowing it to return to battery
position. This withdraws the firing pin.
Check to make sure that the breech end of the barrel projects slightly through the barrel
extension. If there are burs on the breech end of the barrel or in the T-slot, they should
be removed by careful honing.
b.	Retract the bolt until the barrel extension and trunnion are separated approximately
>f6-inch. This puts the locking surfaces of the breech lock and bolt in contact, which is the
position they should assume when a cartridge is chambered.
c.	Check the headspace for tightness by inserting the "GO” end of the gage A351211 in the T-
slot between the recoil plate in the bolt and the end of the barrel. See Figure 75. If the
gage does not go in without being forced, the headspace is too tight; correct the adjustment
by unscrewing the barrel one notch at a time, checking with the gage each time until the
gage enters easily. Check the headspace for looseness in accordance with d.
CAUTION: Never release the firing pin with the gage in place, as to do so may damage
the pin.
Note: The gage may be inserted from either the top or the bottom of the gun. In the event that
the gage is inserted from the bottom, the slack between the bolt and breech lock may be
taken up by inserting a screwdriver between the bolt and the barrel. The gage is made
narrow so this is possible.
d.	Check the headspace for looseness by trying the "NO GO" end of gage A351211 in the
T-slot between the recoil plate in the bolt and the end of the barrel. If the gage does not
go in, and the check for tightness, outlined in c. is satisfactory, the headspace is correct.
If the "NO GO" end of the gage goes in, the headspace is too loose; correct the adjustment
by screwing the barrel into the barrel extension one notch at a time, checking with the
gage each time until the "NO GO" end of the gage will not enter.
e.	Remove the gage, replace the rear right-hand cartridge stop assembly, (if it has been
removed) and release the firing pin.
59
Manipulation
TO CHECK TIMING:
The purpose of this check is to insure that the gun is not fired too early or too late by any of the
various means employed to fire the weapon. In extreme cases of early timing, the gun will
fire two shots and then stop because recoil from the second shot started before the extractor
could engage the next cartridge in the belt. The gun must NOT fire earlier than .116 inches
out of battery. On the other hand, if the gun fires too late while firing automatically, the barrel
extension will strike the trunnion block as the recoding portion moves forward on the counter-
recoil stroke. During automatic firing, the gun must fire before the recoiling parts reach a point
.020 inches out of battery. Only when the first cartridge of a burst is being fired should the firing
pin be released with the recoiling portion in the battery position.
1.	Checking to Insure That the Gun is Not Fired Too Early.
Proceed as follows:
(1)	Adjust the headspace of the gun.
(2)	Cock the firing pin by fully retracting the recoiling portion.
(3)	Raise the cover and retract the bolt slightly inch).
(4)	Insert the .116 inch "No Fire" timing gage, (A351213), between the front of the barrel
extension and the trunnion block with the beveled edge of the gage against the barrel.
(5)	Allow the barrel extension to close slowly on the gage.
(6)	With the gage in place, an attempt should be made to release the firing pin by operating
the solenoid, side plate trigger, or trigger bar. The firing pin should NOT be released. If
the firing pin is released, the solenoid should be adjusted so that release does not take place
with the gage inserted. In cases where the trigger bar or side plate trigger is used, such a
part must be exchanged for another part until one is found which will not release the firing
pin with the gage in place.
2.	Checking to Insure That the Gun is Not Fired Too Late.
Proceed as follows:
(1)	Remove the .116 inch "No Fire" gage and insert in its place the .020 inch "Fire" timing
gage (A351214). With the "Fire" gage in place, an attempt should be made to release
the firing pin. The firing pin MUST BE released.
(2)	If the firing pin is not released, the solenoid must be adjusted or the side plate trigger or
trigger bar must be exchanged for another part until one is found which will release the
firing pin with the .020 inch gage in place.
60
Manipulation
Back Plate Moved Up
To Show Adjustment
Figure 76
TO CHANGE RATE OF FIRE:
It is possible to change the rate of fire by turning the oil buffer tube
The oil buffer body must be partially withdrawn from the receiver
when making this adjustment as the projection on the tube lock is
held firmly in the serrations on the tube when the gun is assembled
to prevent change in the adjustment during firing. The tube is turned
by inserting a screwdriver in the slot in the tube. See Figure 76.
Turning to the right so that the arrow moves toward "C" lessens
the rate of fire and turning to the left toward "O” increases the rate.
Turning the arrow two notches to the right from the "O" position
will give the maximum rate of fire.
The oil buffer should not be turned so far that the arrow points
beyond the ”C" and "O" positions.
TO LOAD:
Before loading, make sure that the bore of the barrel is clear. The end of the belt with the
double metallic links is always fed into the gun. Push the end of the belt of cartridges through
the feed opening as far as it will go and release. With the belt in this position the first cartridge
will be held firmly by the belt holding pawl.
Cock the gun by pulling the bolt to the rear as far as it will go and releasing it. Repeat this
operation and the gun is loaded. Firing will continue as long as trigger action is maintained
and there are cartridges in the belt.
TO UNLOAD:
Raise the cover. Lift the belt with cartridges out of the feed opening, then close the cover. Pull
the bolt to the rear, ejecting the cartridge which remained in the chamber. Check the chamber
and the T-slot in the head of the bolt to be sure all cartridges have been removed.
61
Malfunctions
A "malfunction" is an improper or incomplete action of some part of the gun or its ammunition
resulting in a cessation of fire called a "stoppage."
When describing or analyzing a malfunction it is often important to designate the approximate
position of the recoiling parts when the stoppage occurs. Stoppages may be classified as
follows: a. First Position Stoppage—one which occurs when all recoiling parts are in their
extreme forward or battery position; b. Second Position Stoppage -one which occurs when the
recoiling parts are at any position from just out of battery to the position when the bolt is half-
way back on either the recoil or counter-recoil stroke; c. Third Position Stoppage -one which
occurs when the bolt is in any position from half way back to all the way back on either recoil
or counter-recoil stroke. In the answer sections which follow, the figures in parentheses indicate
the most usual positions of the various stoppages.
When a stoppage occurs, if circumstances permit, the gunner should wait approximately a
minute before doing anything. Then he should examine the ammunition chutes, check the
exposed links and note the position of the cartridges in the feedway without raising the cover.
The position of the bolt should be carefully noted and the bolt held in that position, if possible,
while the cover is opened. Examination may reveal the cause of the malfunction if the bolt is
not allowed to slam forward when the cover is lifted.
Q. What causes failure to feed?
A. The stoppage which results from failure to feed may be caused by any of the fdllowing:
1.	IMPROPER HEADSPACE. (2)
2.	The BELT FEED LEVER MAY BE DEFORMED due to the stud jumping out of the
slot or way in the top of the bolt; or the belt feed stud may be bent or broken causing
the bolt to jam. (2) (3)
3.	The CARTRIDGE MAY NOT ALIGN WITH THE EXTRACTOR due to a bent belt
feed lever giving only limited movement to the belt feed pawl, or the pawl may be de-
formed so that it slides over the cartridge, or the belt feed pawl spring may be weak. (1)
4.	A BROKEN BARREL LOCKING SPRING allowing headspace to change, broken
part jamming mechanism. (2)
5.	The EXTRACTOR CAM MAY BE BROKEN OR WORN. (1) (2)
6.	The EXTRACTOR MAY RIDE TOO HIGH because of a weak or missing extractor
spring in the cover. (1)
7.	A WEAK DRIVING SPRING might fail to drive bolt with enough force to actuate
feed mechanism. (2)
8.	A DEFORMED EXTRACTOR, A BROKEN EXTRACTOR STOP PIN or a
BROKEN OR DEFORMED EJECTOR may permit the cartridge to be out of line
with the chamber. (3)
9.	CARTRIDGES MAY BE TOO SHORT for the extractor to reach the extractor
groove or cannelure of the cartridge. (1)
10.	LOOSE OR WORN MOUNTS may absorb recoil so that driving spring is not fully
compressed. Would appear as Number 7. (2)
62
Malfunctions
Q. What would cause the gun to stop firing with the action completed and a live cartridge in
the chamber?
A. The sear has not released the firing pin extension. This may be due to one or more of the
following:
1.	THE TRIGGER BAR MAY BE BENT so that it fails to operate the sear. (1)
2.	THE TRIGGER BAR MAY BE BENT so as to strike the rear stop before it strikes
the sear, or the rear stop may be too long. (1)
3.	The ENGAGEMENT NOTCH IN THE FIRING PIN EXTENSION may be WORN
or BURRED. (1)
4.	THE SEAR MAY BE BINDING because of burrs on the sear or in the sear recess. (1)
5.	A SHORT FIRING PIN. (1)
6.	A DEFORMED OR BROKEN FIRING PIN. (1)
7.	FIRING PIN BINDING. (1)
8.	HEAVY GREASE such as COSMOLINE around the FIRING PIN SPRING and in
the FIRING PIN HOLE in the bolt. (1)
9.	WEAK OR BROKEN FIRING PIN SPRING. (1)
Q. What would cause insufficient recoil with the result that an empty case would fail to be
extracted and stoppage would occur?
A. This could be caused by any of the following in the air-cooled gun:
1.	THE OIL BUFFER MAY BE SET in the CLOSED POSITION. (2)
2.	GRIT OR BINDING BETWEEN THE BARREL AND THE FRONT BARREL
BEARING, and between the barrel and its bearing in the trunnion. (2)
3.	SIDE PLATES MAY BE SPRUNG together causing binding against the bolt. (2) (3)
Q. What might cause the gun to not quite close or lock on the forward motion so that opera-
tion ceases?
A. This could result from one or more of the following reasons:
1.	TIGHT HEADSPACE. (2)
2.	BURRS IN THE BARREL EXTENSION prevent free movement of breech lock. (2)
3.	THE BELT FEED LEVER MAY BE BENT, thus destroying the feed timing and
preventing the action from closing. (2)
63
Malfunctions
4.	THE BOLT MAY DRIVE FORWARD SLUGGISHLY due to the bolt binding in the
guides of the barrel extension because of burrs or broken parts, or due to a weak
driving spring. (2)
5.	THE BELT FEED SLIDE OR ITS SLOT IN THE COVER MAY BE BURRED,
thus retarding the speed of the bolt. (2)
6.	DIRT RAISES THE BREECH LOCK CAM too high for the lock to ride up on the
cam. (2)
7.	CARTRIDGE DOES NOT SEAT FULLY IN CHAMBER due to burrs on edge of
chamber. (2)
8.	BELT FEED PAWL ARM MAY BE DEFECTIVE so that it strikes the rear cartridge
stop before the action has been completed. (2)
9.	BREECH LOCK CAM BOLT OR SCREW PROJECTS INTO FACE OF CAM to
interfere with breech lock. (2)
Q. What happens if an operating slide bar gets bent?
A. It binds on the side plate and stoppage results. (2) (3)
Q. What causes the face of the bolt and the firing pin to become eroded by hot gases?
A. The primers are being pierced by being struck with a firing pin which is too long, or by a
pin which is sharp or deformed. An undersized firing pin or an oversized firing pin hole
may lead to this condition. (1)
Q. If the bolt jams an empty cartridge case against the lower edge of the barrel or barrel
extension, what may have caused this?
A. The cartridge case is sticking in the T-slot due to:
1.	EXTRUDED PRIMER. (3)
2.	NICKED OR BURRED CARTRIDGE CASE. (3)
3.	BROKEN EJECTOR failing to push last cartridge case down through the T-slot. (3)
4.	ROUGH OR UNDERSIZED T-SLOT. (3)
5.	INSUFFICIENT OR SHORT RECOIL. (3)
Q. It a stoppage occurs, the gun fails to eject, and it is necessary to remove empty case by
hand and occasionally the case is ruptured, what is wrong?
A. This may result from one or more of the following:
1.	EXCESSIVE HEADSPACE. (2)
64
Malfunctions
2.	The T-SLOT IN THE BOLT MAY BE BROKEN OR DEFORMED. (3)
3.	The CHAMBER MAY BE ROUGH, CORRODED OR SCORED. (2)
4.	EXTRUDED PRIMER. (3)
Q. What causes the gun to stop with the bolt forward (stroke completed) and no live cartridge
in the chamber?
A. The gun has failed to extract a cartridge from the belt for one of the following reasons:
1.	The EXTRACTOR MAY BE BENT, BROKEN OR DEFORMED so that it does not
fit the extractor groove of the cartridge. (1)
2.	The BELT LINK MAY BE DEFORMED OR TOO SMALL so that extractor cannot
pull the cartridge out of the belt. (1)
3.	The CARTRIDGES MAY BE TOO SHORT for the extractor to reach the extractor
groove. These short rounds sometimes are the result of the edge of the T-slot of the bolt
striking the end of the cartridge and driving the case forward over the bullet. (1)
4.	EARLY TIMING. (1)
THE FOLLOWING MALFUNCTIONS MAY CAUSE
STOPPAGES IN ANY OF THE THREE POSITIONS:
Q. What happens if cover is not latched or becomes unlatched during firing?
A. Stoppage occurs, probably with breakage of parts.
Q. What might cause the latch not to engage the cover?
A. This might be due to one or more of the following:
1.	The LATCH MAY BE BINDING in the cover or against the top plate or the LATCH
SPRING MAY BE WEAK OR BROKEN.
2.	The SIDES OF THE COVER MAY BE PINCHED or the COVER MAY BE OUT
OF LINE so that it strikes the side plates of the receiver.
3.	The cover may not close because the BELT FEED PAWL ARM IS BENT to inter-
fere with the link stripper.
4.	The LATCH MAY BE WORN OR BROKEN.
5.	The COTTER PIN IS IMPROPERLY INSTALLED in the latch shaft.
65
Malfunctions
Q. What causes a portion of the cartridge case to remain in the chamber?
A. This may be due to EXCESSIVE HEADSPACE. Readjust, and also check for a broken
barrel locking spring.
Q. What causes uncontrolled fire?
A. This may result from the notch on the sear or the notch on the firing pin extension being
worn or deformed. It also may be caused by a weak sear spring. The firing could be stopped
by moving the charging mechanism rearward and holding the bolt out of battery.
Q. What is meant by partial ignition?
A. Sometimes after being fired the powder in the cartridge does not burn completely. Thus the
driving force in back of the bullet is decreased and shooting inaccuracy is the result. In
extreme cases the bullet may remain in the barrel, which is very dangerous.
Q. What is meant by hangfire?
A. A delayed explosion probably caused by defective ammunition or by a light blow by the
firing pin.
Q. What is meant by misfire?
A. If the primer of a cartridge has been indented by the firing pin but is still unfired, this is
called a misfire and is usually due to defective ammunition. Although dangerous while
remaining in the gun, it is not critically dangerous until removed.
Q. If the metal in the primer of the empty cartridge is extruded back next to the firing pin
indent, what causes this, and should anything be done about it?
A. The firing pin is undersize or the firing pin hole in the bolt recoil plate is oversize. This condi-
tion should be corrected as it may lead to pierced primer.
66
Checking Before and After Flight
POINTS TO BE OBSERVED BEFORE A FLIGHT:
The following points must be observed before leaving the ground:
1.	Wipe bore and chamber of gun barrel.
2.	See that adjusting screw is screwed in tight against buffer discs in backplate.
3.	Test functioning of gun by hand, using dummy cartridges.
4.	Test functioning of operating slide, retracting slide or other charging mechanism.
5.	Oil carefully.
6.	See that sight bases are clamped securely in place.
7.	Make sure that ammunition belt is in good condition and properly loaded.
8.	Carefully place belt in ammunition chest and see that metallic belt link chutes are in good
condition and in proper alignment.
9.	Load gun partially or completely as directed.
POINTS TO BE OBSERVED AFTER A FLIGHT:
The following points must be observed as soon after a flight as practicable:
1.	Unload gun completely and remove belt from ammunition chest.
2.	Clean bore and all working parts. If this cannot be done at once, oil carefully to prevent
rust.
3.	Release firing pin spring.
4.	The armorer must get a detailed account from the gunner or pilot of the gun's behavior in
the air. If stoppages have occurred, their cause must be determined and corrected
immediately.
5.	At the first opportunity, dismount gun; clean, oil, and inspect all parts; make needed
repairs and replacements.
6.	On assembling, check operation with dummy cartridges and release firing pin spring after
insuring that functioning and adjustments are correct.
67
Maintenance
CLEANING:
All movable parts of the loading and feeding mechanism should be disassembled and thor-
oughly cleaned with dry-cleaning SOLVENT. The bore should be cleaned with rifle bore
CLEANER. After cleaning, the gun should be given a thin protective coating of lubricating
OIL for aircraft instruments and machine guns. If the gun has not been fired, but is in an alert
condition, it should be cleaned daily with dry-cleaning SOLVENT, thoroughly dried, and a
film of lubricating OIL for aircraft instruments and machine guns, applied. Under no circum-
stances should the gun be allowed to set without cleaning after it has been fired.
When swabbing the bore, the swabbing should be repeated at each cleaning until a clean
flannel patch picks up no foreign matter.
INSPECTION:
The purpose of gun inspection is to determine the condition of a gun and any repairs or adjust-
ments that may be required to keep it in proper operating condition.
During all maintenance operations, the gun should be disassembled and parts thoroughly
cleaned and inspected for wear, scoring, oil leaks, cracks, burrs, carbon, pitting, corrosion,
and rust.
Any burrs or rough edges should be removed by hand honing.
Parts that are broken or worn, or that cannot be satisfactorily improved by hand honing should
be replaced.
All springs should be checked and those not within the specification limits shown on page 79
should be replaced.
The threads on all threaded parts should be checked for burrs or roughness.
All moving parts should be checked to see that they move freely. If any bind is present, the
cause of the binding should be determined and corrected.
After the gun has been thoroughly inspected and cleaned, it should be carefully dried and
covered with a thin film of oil. If the gun is to be stored or shipped, it should be suitably coated
with a rust preventive compound after first cleaning with dry-cleaning SOLVENT.
The following cleaning, preserving and lubricating materials are specified by the Ordnance
Department:
1.	Lubricants OIL—lubricating for aircraft instruments and machine guns.
2.	Cleaners -CLEANER, rifle bore and SOLVENT, dry cleaning.
3.	Rust Preventive COMPOUND -compound, rust preventive, light.
The specifications of the above materials are given in regular U. S. Army Specifications.
68
Ammunition
CLASSIFICATION:
Based upon use, the principal classifications of the ammunition used in this machine gun are:
1.	BALL—For use against personnel and light materiel targets.
2.	ARMOR PIERCING—For use against armored vehicles, concrete shelters, and similar
bullet resisting targets.
3.	TRACER—For observation of fire and incendiary purposes.
4.	INCENDIARY—For incendiary purposes.
Another type provided for special purposes is dummy ammunition for training (cartridges are
inert).
IDENTIFICATION:
Even though the caliber .50 cartridges are not marked or stamped to indicate the type or
model, each may be identified as described below. In general, the only stamping on the cart-
ridge is the manufacturer's initials and year of loading which appear on the base of the
cartridge case. On lots manufactured prior to 1940, "Cal 50” is also stamped on the base of
the cartridge case. However, the marking on all original packing containers, both boxes and
cartons, clearly and fully identifies the ammunition except as to grade. In addition to the
marking, colored bands painted on the ammunition boxes, and on carton labels, provide a
ready means of identification as to type.
When removed from their original packing containers, cartridges may be identified, except as
to ammunition lot number and grade, by physical characteristics as described below. Care
should be taken not to confuse these original markings with any subsequent markings made
with lithographic marking ink, which is used for an entirely different purpose.
1.	BALL Cartridge, caliber .50, М2 and Ml, is the standard ball ammunition for this
weapon. Cartridge, ball, caliber .50, Ml923, is limited standard. All caliber .50
ammunition have bullets with gilding metal jackets (copper colored).
2.	ARMOR PIERCING—All models of caliber .50 armor piercing ammunition may be
distinguished by the nose of the bullet which is painted black for a distance of approxi-
mately % inch from the tip.
3.	TRACER—Caliber .50 tracer ammunition may be identified by the nose of the bullet
which is painted red for a distance of approximately % inch from the tip.
4.	INCENDIARY—Caliber .50 incendiary ammunition may be identified by the nose of
the bullet which is painted light blue for a distance of approximately % inch from the tip.
5.	DUMMY—Caliber .50 dummy cartridge may be identified by a hole in the body of
the cartridge case.
LOT NUMBER:
When ammunition is manufactured, an ammunition lot number which becomes an essential
part of the marking is assigned in accordance with pertinent specifications. This lot number is
69
Ammunition
marked on all packing containers and on the identification card inclosed in each packing
box. It is required for all purposes of record, including grading and use, reports on condition,
functioning, and accidents in which the ammunition might be involved. No lot other than that
of current grade appropriate for the weapon will be fired.
Since it is impracticable to mark the ammunition lot number on each individual cartridge,
every effort should be made to maintain the ammunition lot number of the cartridges once
they are removed from their original packing. Cartridges which have been removed from
original packing and for which the ammunition lot number has been lost are placed in grade
3. Therefore, when cartridges are removed from their original packings they should be marked
so that the ammunition lot number may be preserved.
IDENTIFICATION CARD:
An identification card, approximately 7x15 inches, showing the quantity, type, caliber, model,
ammunition lot number, and manufacturer is sealed inside the metal liner on top of the
ammunition in each box.
MARKING:
Color bands painted on the sides and ends of the packing boxes further identify the various
types of ammunition. The following color bands are used:
Cartridge, armor piercing...........Blue on yellow
Cartridge, ball ....................Red
Cartridge, ball and tracer,
in metallic link belt..............Composite band of yellow, red, and
green stripes (yellow on left, red
in center, green on right)
Cartridge, blank	Blue
Cartridge, dummy.................... Green
Cartridge, tracer............. ...	Green on yellow
Cartridge, incendiary .	.	Red on	yellow
Carton labels are similarly marked to show the quantity, type, caliber, model, ammunition lot
number, and manufacturer. Color stripes similar to those on the packing boxes are also
printed on the labels, except that for blank ammunition the label itself is blue and for dummy
ammunition it is green.
The number of hits made upon a target by a certain machine gun or group of machine guns
when others are firing upon the same target is sometimes determined by coating the tips of the
bullets with lithographic ink. The bullets from each weapon or group of weapons are coated
with a distinctive color of ink which, upon striking the target, leaves a smear indicating the
source of fire. Cartridges which have been so coated must have the ink removed before return
to storage.
70
Ammunition
CARE, HANDLING AND PRESERVATION:
Ammunition boxes should not be opened until the ammunition is required for use. Ammunition
removed from the airtight container, particularly in damp climates, is likely to corrode, thereby
causing the ammunition to become unserviceable.
The ammunition should be protected from mud, sand, dirt, and water. If it gets wet or dirty,
wipe it off at once. Verdigris or tight corrosion should be wiped off. Cartridges should not be
polished, however, to make them look better or brighter.
The use of oil or grease on cartridges is prohibited.
Do not fire cartridges with loose bullets or other defects.
Ammunition should not be exposed to the direct rays of the sun for any length of time. This is
likely to affect seriously its firing qualities.
Whenever cartridges are taken from cartons and loaded into belts, the latter will be tagged so
that the ammunition may be identified as to lot number. Tagging is necessary in order to
preserve the grade of the ammunition.
STORAGE:
Whenever practicable, small arms ammunition should be stored under cover. This applies
particularly to tracer ammunition which is subject to rapid deterioration if it becomes damp,
and may even ignite spontaneously. When necessary to leave small arms ammunition in the
open, raise it on dunnage at least 6 inches from the ground and cover it with a double thick-
ness of paulin. Suitable trenches should be dug to prevent water flowing under the pile.
If practicable, tracer ammunition should be stored separately from other ammunition.
If tossed into or placed in a fire, small arms ammunition does not explode violently. There are
small individual explosions of each cartridge, the case flying in one direction and the bullet
in another. In case of fire, keep personnel not engaged in fighting the fire at least 200 yards
from the fire and have them he on the ground. It is unlikely that the bullets and cases will fly
over 200 yards.
AUTHORIZED ROUNDS:
The following ammunition of appropriate grade is authorized for use in the Browning Machine
Gun, caliber .50, М2, aircraft:
Cartridge, armor piercing, cal. .50, Ml and М2
Cartridge, ball, cal. .50, Ml and М2
Cartridge, dummy, cal. .50, Ml............
Cartridge, dummy, cal. .50, М2 ....
Cartridge, tracer, cal. .50, Ml................
Cartridge, incendiary, cal. .50, Ml. ...
Standard
Standard
Limited Standard
Standard
Standard
.......Standard
71
Complete Parts List
BACK PLATE GROUP
Quantity
Description	Piece No. Per Gun
Back Plate Assembly (With Horizontal Buffer).......... C64311	1
Adjusting Screw..................... ....	............ A152834	1
Adjusting Screw Plunger.................................... A152839	1
Adjusting Screw Plunger Spring............................. A9300	1
Buffer Disc............. .................................. A152835	22
Buffer Plate............ ..................................A152869	1
Back Plate Filler Piece Pin................................ A9275	1
Back Plate Filler Piece ................................... A152753	1
Back Plate Latch Pin ...................................... A9275	1
Back Plate Latch	........ .......................... B8949	1
Back Plate Latch Spring.................................... A9356	1
Back Plate Latch Lock Pin.................................. A13581	1
Back Plate Latch Lock................................... В147464	1
Back Plate Latch Lock Spring	......... B243607	1
Back Plate Latch Lock Spring Pin ...	................. A262799	1
Lower Filler Piece Pin	..................... A152854	2
Lower Filler Piece	........................ A152750	1
Back Plate ...	D35276	1
Cotter Pins:
Latch Lock Pin and Lower Filler Piece Pin x %"). .	BFAXIBB 3
72
Complete Parts List
ALTERNATE FEED BOLT GROUP
Description	Piece No.	Quantity Per Gun
Bolt Stud	A13424	1
Bolt Handle	B8583	1
Driving Spring Rod Assembly With Springs .	. C64305	1
Driving Spring Rod Collar Stop Pin		 A152898	1
Driving Spring Rod Collar	. A152900	1
Driving Spring, Outer	. В147509	1
Driving Spring, Inner		 B147510	1
Driving Spring Rod Assembly	...		 C64304	1
Driving Spring Rod Retaining Pin	A152899	1
Driving Spring Rod Head	A152901	1
Driving Spring Rod		В147508	1
Extractor Assembly	B8959	1
Ejector Pin	. .	A9273	1
Ejector	B9732	1
Ejector Spring	. A9523	1
Extractor	C4065	1
Bolt Switch	...	C4062	1
Cocking Lever Pin	В147762	1
Cocking Lever ...	B9718A	1
Sear Stop Assembly	B8788	1
Sear Stop	B9723	1
Sear Stop Pin	A9381	1
Sear Slide	A13595	1
Sear	C4067	1
Sear Spring	A9524	1
Firing Pin Extension Assembly	. B8976	1
Firing Pin Spring Stop Pin	A9382	1
Firing Pin Spring	. . A9353	1
Firing Pin Extension	B8946	1
Firing Pin	B17171	1
Alternate Feed Bolt Assembly	В147463	1
Bolt Switch Stud	A13529	1
Recoil Plate	A152858	1
Extractor Stop Pin	A9385	1
Alternate Feed Bolt	D28256	1
73
Complete Parts List
OIL BUFFER BODY AND OIL BUFFER GROUP
Description	Piece No.	Quantity Per Gun
Oil Buffer Tube Lock Assembly. .	...	. . .. C145325	1
Oil Buffer Tube Lock Rivet	...	...	. . A312673	2
Oil Buffer Tube Lock Body		B284606	1
Oil Buffer Tube Lock Latch			. . . B284610	1
Accelerator Pin Assembly....				 B8790	1
Accelerator Pin 			 A9276	1
Accelerator Pin Spring		 A9357	1
Accelerator ..				 C8141	1
Oil Buffer Body Spring Lock		 . . .		 A9266	1
Oil Buffer Body Assembly				 C3941	1
Breech Lock Depressor				 B9712	2
Breech Lock Depressor Rivet				 A9283	2
Oil Buffer Guide 	 ...	. .	. B9715	2
Oil Buffer Body	 ....		 C8063	1
Oil Buffer Assembly	 		. . C4077	1
Oil Buffer Spring Guide Assembly		B8782	1
Oil Buffer Spring Guide Key ...	. . ..	A9520	1
Oil Buffer Spring Guide		A9518	1
Oil Buffer Spring. .	...				 B9832	1
Oil Buffer Tube Fillar Screw 			 A9361	2
Oil Buffer Tube	 		.. .	C8146	1
Oil Buffer Piston Head Nut Pin	x	A9380	1
Oil Buffer Piston Head Nut	 ...	A9267	1
Oil Buffer Piston Valve Assembly ...			. . . B8969	1
Oil Buffer Piston Valve Key		. . A9784	2
Oil Buffer Piston Valve			 B17175	1
Oil Buffer Piston Rod Head 			 B17169	1
Oil Buffer Tube Cap.	...	....		 B9731	1
Oil Buffer Relief Valve Screw			. . . A9360	1
Oil Buffer Relief Valve Spring 		. .. A9393	1
Oil Buffer Relief Valve				A9528	1
Oil Buffer Packing Gland Spring		. ..	A153163	1
Oil Buffer Packing Gland Washer			. . A153161	1
Oil Buffer Gland Packing			 A153162	1
Oil Buffer Packing Gland Plug 			 A9277	1
Oil Buffer Piston Rod Assembly .				 B8763	1
Oil Buffer Piston Rod Pin			 A9379	1
Oil Buffer Piston Rod				 B9830	1
74
Complete Parts List
BARREL AND BARREL EXTENSION GROUP
Quantity
Description	Piece No.	Per Gun
Barrel	D35348A	1
Barrel Locking Spring ...	. B8908	1
Breech Lock Pin Assembly			. ..	B8784	1
Breech Lock Pin		 A9274	1
Breech Lock Pin Spring		A9357	1
Breech Lock	...	. .	B8925	1
Barrel Extension Assembly. .	...			C4082	1
Barrel Extension Shank Lock Pin ...	. ..	A9268	1
Barrel Extension Shank		. B9728	1
Barrel Extension ...	D28254	1
75
Complete Parts List
RECEIVER AND BARREL JACKET GROUP
Description	Piece No.	Quantity Per Gun
Belt Holding Pawl Pin Assembly		.. .	B8963	2
Belt Holding Pawl Pin Spring		.. A13497	2
Belt Holding Pawl Pin Head 			 	 A152567	2
Belt Holding Pawl Pin			 B8917	2
Belt Holding Pawl			 B261098	1
Belt Holding Pawl Spring ..				 A153146	2
Front Cartridge Stop				A13539	1
Link Stripper				#A13541	1
Rear Cartridge Stop 	 .	. .		#A13540	1
Rear Right-Hand Cartridge Stop Assembly		 C77409	1
Cartridge Aligning Pawl Pin...		 A13612	1
Cartridge Aligning Pawl Plunger	. . .		 A13611	1
Cartridge Aligning Pawl Spring ... .		 A13613	1
Cartridge Aligning Pawl		. . B8975	1
Rear Right-Hand Cartridge Stop		 C77408	1
Cover Detent Pawl Assembly		 B8515	1
Cover Detent Pawl Guide	.. A13001	1
Cover Detent Pawl ..	B8927	1
Cover Detent Pawl Spring..		 .	A13520	1
Trunnion Block Cover Pin	....	A13546	2
Trunnion Block Cover	. .		 A13588	1
Switch Pivot Nut	....	. A13556	1
Switch	 .	. .	.	.. B147461	1
Switch Spring	. B8943	1
Breech Lock Cam Bolt Nut	. . . A152939	1
Breech Lock Cam Bolt		 A152938	1
Breech Lock Cam	В147583A	1
Top Plate Cover Screw	A13608	3
Top Plate Cover	B8939	1
Trigger Bar Pin Assembly	B8683	1
Trigger Bar Pin Lock	B9786	1
Trigger Bar Pin Key	A9519	1
Trigger Bar Pin	A20708	1
Trigger Bar	B8944	1
Trunnion Adapter	C4052A	1
Trunnion Block Shim	B8951	1
Trunnion Block Lock 		. . A13565	1
Trunnion Block Lock Spring	A13566	1
Breech Bearing Lock Screw	A152829	1
#For Right-Hand Feed
76
Complete Parts List
RECEIVER AND BARREL JACKET GROUP (Cont.)
Quantity
Description	Piece No. Per Gun
Barrel Jacket Assembly With Front Bearing........................ C64290	1
Front Barrel Bearing Screw .................................. A13655	2
Front Barrel Bearing......................................... B8910	1
Barrel Jacket Assembly....................................... C4047	1
Breech Bearing........................................... B8921	1
Barrel Jacket............................................ D28255	1
Receiver Assembly................................................ D35480	1
Left-Hand Side Plate Assembly................................ C4085	1
Left-Hand Belt-Holding Pawl Bracket...................... C4059	I
Extractor Cam ........................................... A9373	1
Bolt Stop ............................................... A9392	1
Left-Hand Side Plate..................................... D28262	1
Right-Hand Side Plate Assembly............................... C4086	1
Right-Hand Belt-Holding Pawl Bracket..................... Bl28730	1
Right-Hand Side Plate ................................... D28261	1
Top Plate Assembly........................................... C4087	1
Top Plate Bracket Assembly ........... .................. В147817	1
Top Plate Bracket................................... C4070	1
Top Plate Bracket Stud ............................. A9367	1
Front Trigger Bar Stop ............................. A9391	1
Rear Trigger Bar Stop.................................... A9390	1
Bolt Latch Bracket....................................... C4061	1
Top Plate................................................ D28263A	1
Bottom Plate ................................................ D28257A	1
Trunnion Block Assembly...................................... C4076	1
Bunter Plug ............................................. A13572	1
Trunnion Block........................................... D28264	1
Cotter Pins:
Cover Detent Pawl and Switch Pivot (%" x %") ................ BFAXIBE	2
Breech Lock Cam Bolt (1/6" x 1И")............................ BFAXIDK	1
Trunnion Block Lock (%2" x %")............................... BFAXICE	1
Rivets:
Belt Holding Pawl Bracket (L. H. Long)............................ A13598	3
Extractor Cam	.................................. A9501	2
Belt Holding Pawl Bracket (R. H. Long)....................... A13598	3
Belt Holding Pawl Bracket (R. H. Short)...................... A13698	2
Top Plate Bracket ........................ .................. A9292	2
Bolt Latch Bracket (Short).......... ........................ A13522	5
Bolt Latch Bracket (Long) ................................... A13521	4
Top Plate.................................................... A9500	16
Bottom Plate	......................................... A9387	16
Trunnion Block (Short) .	.................................. A9799	18
Trunnion Block (Long)........................................A13558	4
77
Complete Parts List
COVER AND BELT FEED GROUP
Description	Piece No.	Quantity Per Gun
Belt Feed Lever	...		 C64278	1
Belt Feed Lever Plunger	. .	A13515	1
Belt Feed Lever Plunger Spring	...	A13516	1
Belt Feed Pawl Pin Assembly 		B8962	1
Belt Feed Pawl Pin Spring	. .	A9357	1
Belt Feed Pawl Pin	A13519	1
Belt Feed Pawl Spring ....	A9351	1
Belt Feed Pawl Arm			. . B8914	1
Belt Feed Pawl Assembly. .	. B8961	1
Belt Feed Pawl Arm Pin	. . . A13517	1
Belt Feed Pawl Arm Locating Pin		 A13518	2
Belt Feed Pawl	... .	B8913	1
Belt Feed Slide Assembly	....	. B261110	1
Belt Feed Pawl Spring Stud	. . .	A9363	1
Belt Feed Slide	. . A147756	1
Cover Latch Spring	. B8931	1
Cover Extractor Spring	B9741	1
Cover Latch Shaft Washer		A13545	1
Cover Latch Shaft Assembly	B8964	1
Cover Latch Shaft Lever Pin	A13587	1
Cover Latch Shaft Lever	.	.	.	. .	. A13586	1
Cover Latch Shaft Key	. .	A13544	1
Cover Latch Shaft	. .	. B8930	1
Cover Latch	. .	..	. . B8928	1
Cover Pin	A9271	1
Cover Assembly	. C4081	1
Cover Extractor Cam			C64279	1
Cover Latch Spring Stud	A9366	1
Belt Feed Lever Pivot Stud Washer	A9398	1
Belt Feed Lever Pivot Stud Pin	A9384	1
Belt Feed Lever Pivot Stud	A9395	1
Cover Extractor Spring Stud	. A9365	1
Bracket	A152752	1
Cover	D28258	1
Cotter Pins:		
Belt Feed Lever Pivot Stud (%2" x ^")	BFAXICE	1
Cover Latch Shaft (K6" x %"')	. BFAXIBB	1
Cover Pin	x 5^")	. BFAXIDD	1
Rivets:		
Cover Extractor Cam	A9282	3
Bracket, Countersunk Head (^" x 1*4")	A152980	2
78
Complete Parts List
SPRINGS
Name	Piece No.	Free Outside Diameter (Inches)	Wire Diameter (Inches)	Free Length (Inches)
Back Plate Latch Spring	A9356	0.240	0.042	1.125
Adjusting Screw Plunger Spring	A9300	0.153	0.027	0.665
Firing Pin Spring	A9353	0.295	0.059	3.218
Sear Spring	. A9524	0.241	0.033	0.560
Ejector Spring	A9523	0.143	0.031	0.340
Driving Spring (Inner)	. B147510	0.287	0.039	22.000*
Driving Spring (Outer)	В147509	0.403	0.053	22.000*
Oil Buffer Packing Gland Spring	. A153163	0.424	0.062	0.385
Oil Buffer Relief Valve Spring	. A9393	0.208	0.034	0.450
Oil Buffer Spring	B9832	1.448	0.125	5.875
Belt Feed Pawl Spring .	A9351	0.345	0.038	1.050
Belt Feed Lever Plunger Spring	A13516	0.152	0.031	0.718
Cartridge Aligning Pawl Spring	A13613	0.197	0.026	0.750
Belt Holding Pawl Spring	A153146	0.181	0.026	0.796
Cover Detent Pawl Spring	A13520	0.300	0.055	0.843
Trunnion Block Lock Spring	A13566	0.321	0.031	0.937
’Maximum 23 0 Inches. Minimum 21.0 Inches.
79
Heavy Barrel, М2, Gun
The Browning Machine Gun, Caliber .50, НВ, М2 is an air-cooled gun having a much heavier
barrel than has the aircraft gun. Its general appearance may be noted in Figure 77.
Figure 77. Browning Machine Gun, Caliber .50, НВ, М2
The gun is normally fired in short bursts or in rapid single shots, and when used in this manner
firing may be continued for an appreciable length of time because the heavy barrel retards
overheating.
GENERAL DATA (Approximate)
Weight of Gun
Weight of Barrel
Length of Barrel
Number of Lands
Twist Right Hand
Overall Length of Gun
Muzzle Velocity
Rate of Fire
Maximum Range
81 lbs.
27.5 lbs.
45 inches
8
1 turn in 15 inches
65.125 inches
2,935 ft. per sec. (2,000 mi. per hr.)
400 to 500 rounds per minute
7,200 yards (4.1 miles)
In place of the barrel jacket assembly on the aircraft gun this gun uses a short, perforated barrel
support. The trunnion adapter of the aircraft gun is not used. The heavy barrel is removed from
the gun by unscrewing it from the barrel extension and withdrawing it toward the front. This
permits removing a hot barrel and installing a cool one without disassembling the remaining
mechanism of the gun. The handle assembly, shown just ahead of the barrel support, is used
for carrying the gun or as a means of turning the barrel when assembling, disassembling or
adjusting the headspace. It is moved to one side or down when the gun is being fired.
CAUTION: Disengage handle before turning so that headspace adjustment will not be altered.
The firing mechanism is modified somewhat from that included with the aircraft gun. A bolt
latch is provided to permit the gun to be fired semi-automatically. It also serves to hold the bolt
to the rear in order to keep the cartridge out of the hot chamber when firing has been suspended.
80
Heavy Barrel, М2, Gun
Figure 78. Bolt Latch Mechanism
The bolt latch is forced downward by the bolt latch spring. As the bolt reaches its rearward
position, the bolt latch engages a notch on the upper rear surface of the bolt and holds the bolt
to the rear, thus causing the gun to cease firing. The counter-recoil stroke is completed by
pressing down on the bolt latch release which is pivoted in the back plate. This raises the bolt
latch from the bolt notch and allows counter-recoil to take place. Providing a cartridge is in the
chamber, firing will be resumed when trigger action is supplied. If the bolt latch release is held
down manually, or if it is locked down by the lock on the buffer tube sleeve, the gun will fire
automatically. However, if the bolt latch release is pressed down but not retained in that
position, the gun will fire only once when trigger action is given.
The back plate spade grip assembly is similar to that used on the Aircraft Gun except for the
addition of the buffer tube sleeve assembly and the bolt latch release and spring.
Since the recoiling portion is much heavier than in the aircraft gun, its rearward motion is not
quite so rapid; therefore, it is unnecessary to have as much restriction in the oil buffer on the
recoil stroke. Accordingly, the oil buffer piston valve assembly, the gland packing, gland
washer, gland spring, oil and oil filler screws are omitted from the heavy barrel gun.
With these exceptions and a few changes in the accessories supplied, such as front and rear
sights, the heavy barrel gun is identical with the aircraft gun.
81
Water Cooled, М2, Gun
The Browning Machine Gun, Caliber .50, М2, water-cooled, has a water jacket surrounding
the barrel for the purpose of preventing barrel overheating when firing for prolonged periods.
The general appearance of this gun may be noted in Figure 79.
Figure 79. Browning Machine Gun, Caliber .50, М2, Water Cooled
The water jacket contains ten quarts of water, and is kept filled by a hand pump from an
auxiliary water chest which has a capacity of about eight gallons. During firing, heat absorbed
from the barrel changes some of the water to steam. This is removed from the jacket with the
water being returned through the jacket outlet to the water chest.
GENERAL DATA (Approximate)
Weight of Gun (with water)
Weight of Gun (without water).
Weight of Barrel
Length of Barrel
Number of Lands
Twist—Right Hand
Overall Length of Gun
Muzzle Velocity
Rate of Fire.
Maximum Range
121.5 lbs.
100.5 lbs.
16 lbs.
45 inches
8
1 turn in 15 inches
65.93 inches
2,935 ft. per sec. (2,000 mi. per hr.)
600 to 750 rounds per minute
7,200 yards (4.1 mi.)
As in the aircraft gun the barrel recoils. The water jacket, however, is stationary since it is
screwed on to the trunnion block. Thus packing glands must be provided near the breech and
muzzle ends of the barrel to prevent water from escaping from the jacket where the barrel
slides in and out of the jacket.
With these exceptions and a few changes in the accessories supplied, such as front and rear
sights, the water cooled gun is identical with the aircraft gun.
82
DETAILS OF BROWNING MACHINE GUN C A L I В E R .5 0, M 2, A I R C R A F T BASIC
млниглстикео by
AC SPARK PLUG DIVISION, General Motors Corporation, FLINT, MICHIGAN
FRIGIDAIRE D I V I S I О N, General Motors Corporation, DAYTON, OHIO